has developed the Quiet Climb System, an automated avionics
feature for quiet procedures that involve thrust cutback after
takeoff. By reducing and restoring thrust automatically, the
system lessens crew workload and results in a consistently
quiet footprint, which helps airlines comply with restrictions
and may allow for an increase in takeoff payload..
With higher density populations
surrounding airports throughout the world, the sound of airplanes
has become an issue of increasing importance in recent years. Noise-abatement
requirements and procedures imposed by local airport authorities
have affected airline operations in many ways, resulting in restricted
hours of operation, required weight offloads, fines, and surcharges.
Airplane and engine manufacturers
have been successful in producing quieter airplanes, but more stringent
noise-abatement requirements and the high cost of engine modification
have prompted the industry to consider additional ways to decrease
airplane sound in communities.
One alternative is a
maneuver in which the flight crew takes off with full takeoff power,
climbs rapidly, and then cuts the thrust manually to a predetermined
value at a specified cutback altitude. The airplane continues to
climb, albeit at a much slower rate, until it is high enough that
sound in the community is not an issue. The crew then adds more
power to continue flight.
One potential problem
with this maneuver is that the pilot must cut back and restore engine
thrust manually at the correct altitudes. The Boeing Quiet Climb
System (QCS), which is selected during the takeoff procedure, automatically
reduces and restores engine thrust at the specified altitudes, thereby
reducing pilot workload.
In an effort to standardize
noise-abatement procedures, the Federal Aviation Administration
(FAA) has issued advisory guidelines that define departure profiles,
including the minimum thrust required and the cutback altitude.
This article discusses
of sound measurements.
of the Boeing QCS on sound in communities.
FAA ADVISORY GUIDELINES
In 1993, the FAA issued
advisory circular AC91-53A, Noise Abatement Departure Profiles,
which standardizes procedures by defining acceptable criteria for
speed and minimums for thrust cutback settings and altitudes for
various airplane takeoff configurations.
Minimum thrust cutback.
The minimum thrust cutback represents the minimum level of thrust
that would ensure a sufficient climb gradient if an engine were
to fail. This thrust value is determined by the number of engines
on the airplane. On a two-engine airplane, the minimum thrust cutback
ensures an engine-inoperative climb gradient of 1.2 percent. If
one engine fails after cutback, the thrust from the operating engine
must maintain a climb gradient of at least 1.2 percent. On three-engine
and four-engine airplanes, the minimum thrust cutbacks are engine-inoperative
climb gradients of 1.5 percent and 1.7 percent, respectively.
Zero percent gradient
Under certain conditions, the advisory circular also allows a thrust
cutback that maintains a zero percent engine-inoperative climb gradient.
This type of cutback is permitted for airplanes with avionics systems
that can detect engine failure and automatically increase the thrust
on the remaining engine or engines to a value that maintains the
minimum climb gradient. These minimum climb gradients are 1.2 percent
on a two-engine airplane, 1.5 percent on a three-engine airplane,
and 1.7 percent on a four-engine airplane.
The advisory circular also specifies that the minimum altitude at
which the thrust can be reduced, or cut back, is 800 ft above ground
THE BOEING QCS
Boeing developed the
QCS, an advanced avionics feature, to directly assist flight crews
in flying the quiet departure profiles defined in the advisory circular.
The QCS controls thrust reduction and restoration automatically,
thereby eliminating the need for manual control and ensuring consistency.
During the takeoff checklist
procedure, the pilot selects the QCS and enters the altitudes at
which thrust should be reduced (greater than or equal to 800 ft
AGL) and restored. With the autothrottle system engaged, the QCS
reduces engine thrust when the cutback altitude is reached to maintain
the optimal climb angle and airspeed. When the airplane reaches
the chosen thrust restoration altitude (typically 3,000 ft AGL),
the QCS restores full climb thrust automatically. As such, QCS reduces
pilot workload during a phase of flight that already is task intensive.
QCS incorporates multiple
safety features and will continue to operate even with system failures.
If a system failure does occur, there are several methods for exiting
QCS. In the most common method, the pilot selects the takeoff/go-around
switches on the throttle control stand. The pilot can take control
of the throttles easily by disconnecting the autothrottle and controlling
the thrust manually, as appropriate.